Monster Bounces

Make a tennis ball bounce much higher than the height you dropped it from using a neat piece of physics.

What you need

A tennis ball or another light bouncy ball

A basket ball or another large heavy very bouncy ball.

What to Do

Hold the tennis ball directly on top of the basket ball, and then try to drop both balls together as smoothly as possible onto a hard surface. Make sure you stay a long way away from anything delicate!

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What may Happen

You should find that the tennis ball bounces far higher than the height it was dropped from.

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What is going on?

If you drop a ball it will only ever bounce up nearly as high as it was dropped from. However if you drop two balls more interesting things can happen. The basketball hits the ground and bounces first so the the tennis ball hit the basketball coming back up. This is like hitting it with a tennis racket so it bounces off much faster than it was coming down and it flies up into the air.

The two balls start dropping together	The balls speed up as they fall.	The basketball bounces first so the tennis ball hits the basket ball coming upwards.	The tennis ball bounces off the ball like off a tennis ball.

Make a camera from a lemonade bottle

Build a camera or if you prefer a magnifying glass, out of some old rubbish and a little water

What you need

	A 2 litre lemonade (soda) bottle		A pair of scissors
	An interesting light in the ceiling

What to Do

Cut a roughly circular piece of plastic out of the lemonade bottle, from the part of the bottle just below the neck that is curving in two directions at once. You should end up with a dish shaped piece of clear plastic.

Turn on the light.

Put a piece of white paper on the floor.

Pour a little water into the dish shaped piece of plastic.

Hold it still about 15cm above the paper - move the bowl up and down

Do you see anything on the paper?

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What may Happen

You should see an image of the light projected onto the paper on the floor - you have built a camera. If you are too close, too far away or if you squeeze the bowl out of shape the image will go fuzzy, if you look through the water it should act like a magnifying glass.

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What is going on?

When you pour water into the plastic bowl the water takes on the shape of the bowl, because it is a liquid.

This shape is roughly the same as the shape of a lens or a magnifying glass. This means that most of the light leaving one point on the light which hits the water will be bent so that it goes through a single point on the other side of the lens.

Each point where the light starts will end up in a different point below the lens, so if you put a piece of paper there all the points projected onto the paper will form an image.

If you want to find out more about this see the Camera Obscura and Pinhole Camera experiments.

Why is the image not perfect?

Surface tension means that the top surface of the water is actually not flat, which will mean that light is sent in the wrong directions at the edges. Also, the shape of the bottle is only roughly the right shape to make a lens, it is designed to hold lemonade not focus light, but it does a pretty good job despite this.

Measuring the Speed of Light

Measure the highest speed possible in this universe, just using objects you could find in your kitchen.

What you need

	A microwave		A plate or shallow bowl
	Four slices of bread		Some butter or margarine

What to Do

Take the turntable out of the microwave.

Put an upside down plate over the rotating parts.

Spread the butter evenly on all 4 pieces of bread, right out to the edges.

Lay the four slices on another plate or the turntable plate, and add some butter at the joints.

Put this in the microwave, heat until the butter starts to melt (10-15 seconds) it is probably worth checking every 4-5 seconds

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What may Happen

You should find that the butter melts in splodgy bands. If you measure the distance between these and multiply it by two you will find the wavelength of the microwaves that the microwave oven is using to cook with.

If you look on the back of your microwave oven you should see a number in GHz (billion vibrations a second) or MHz (million vibrations a second), this is the number of waves the microwave produces every second - the frequecy.

If you multiply the wavelength you found by this frequency you will find the speed of light.

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What is going on?

A microwave oven works by producing microwaves - these are a type of electromagnetic wave just like radio waves, infra red or visible light. They are produced by a device called a magnetron and leave from a hole in the right.

The microwaves will reflect back and forth from the two sides of the metal oven.

The wavelength of the microwaves is tuned to produce a standing wave. This is where you get two waves, one going in each direction, these interact to make some areas where there is a huge vibration and others where there are none.

This means that there are places where the microwaves are very intense, where the molecules will be vibrated very powerfully so heated strongly. Others where the microwaves are weak. These areas separated by half a wavelength. This is why there is a turntable in a microwave oven, otherwise parts of your food will be overcooked and others will still be raw.

If you put the buttered bread in the microwave the butter will first melt where the microwaves are the most intense, so the distance between these will be half a wavelength.

So what has the wavelength got to do with the speed of light?

Your microwave oven is producing microwaves vibrating at a certain frequency - this is written on the back of your microwave.

The wavelength depends on how rapidly the wave is vibrating (the frequency) and how fast it is moving - as it is a type of light, the speed of light.

	If you have a wave traveling at a fixed frequency (a certain number of vibrations a second).
	If it is moving more slowly it will travel less far between vibrations so the wavelength will be less
	If it moves faster then the wavelength will be longer.

So this means that if the wave has a frequency of 100Hz it will travel it's wavelength 100 times in a second. So a waves speed is frequency × wavelength.

Because microwaves are a type of light you have just measured the speed of light in your kitchen!

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Written by Dave Ansell

Strange Glows from Sugar

How to make strange unearthly glows by torturing sugar cubes...

What you need

	Some Lumpy Sugar, or sugar lumps.
	A Pair of Pliers
	A very very dark room

What to Do

Get the Pliers and a lump of sugar somewhere you can find them in the dark.

Turn off the lights and wait for at least 2 minutes - this will make your eyes much more sensitive.

Carefully crush a lump of sugar in your pliers. - watch the sugar and see what happens.

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What may Happen

With any luck you should see little flashes of blue-green light as you crush the sugar.

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What is going on?

When you crush the lump of sugar, you are fracturing sugar crystals, sugar crystals are slightly asymetric and, when you apply pressure, on a molecular scale some areas are slightly positive and some slightly negative.

if the crystal breaks it will sometimes have more positive charge on one side of the fracture and more negative on the other.

If the two halves of the crystal are pulled apart you are seperating the positive and negative charges which takes quite a lot of energy (a bit like pulling two magnets apart). Voltage is a measure of how much energy each charge has got so the voltage builds up.

At some point this voltage gets large enough for the charge to flow through the air as a spark. In order to do this it has to rip air molecules apart, giving them lots of energy. They release some of this as light which creates the strange glows.

This effect is called triboluminescence, and similar effects can be seen in lots of different materials, from sellotape to flint pebbles on the beach

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Written by Dave Ansell